Ultramarathoners Show Us There's a Metabolic Limit

Elite ultramarathon runners can push their bodies to incredible extremes, but a new study suggests even they can't break through a fundamental metabolic ceiling. Researchers tracked 14 endurance athletes—mostly professionals, including 10 ultramarathoners—over the course of a year, using advanced metabolic testing. They found these athletes could reach up to seven times their basal metabolic rate (BMR)—the rate at which the body uses energy while at rest—during competition, burning as many as 11,000 calories a day, per Scientific American. But this peak was short-lived. Over periods of 30 and 52 weeks, energy expenditure dropped to around 2.5 times their BMR—a limit scientists previously suggested can't be exceeded for long.

For a lean, 150-pound person, this equates to burning about 3,750 calories a day, a rate the average jogger would never approach, scientists say. "It takes running about 11 miles on average a day for a year to achieve 2.5 times BMR," says Massachusetts College of Liberal Arts anthropologist Andrew Best, co-author of the study published Monday in Current Biology, per a release. This work "was really meant to test this 2.5-times hypothesis in a population of people who should be uniquely suited to potentially break that ceiling," he adds, per Scientific American, noting he was "surprised" that even the most rugged endurance athletes in the study stayed under the suspected limit.

Experts suggest that sustaining activity near this metabolic ceiling takes a toll, potentially slowing digestion, weakening immunity, and even shrinking brain tissue temporarily. But others believe the true limit could be higher—decades-old research on Tour de France cyclists hinted at a limit of four to five times a person's BMR—especially as advances in sports nutrition allow athletes to consume more calories. As one expert asks, "Is it an absolute human limit that we will never be able to surpass? Or is it a historically contingent limit that we just haven't passed yet?" For now, those remain open questions.